Receptor feature extraction not defined to be parcellation-sensitive about siibra-python

[dickscheid]

What I suggest is the following solution for RegionFeatures:

• load the data only once, when its feature extractor is called the first time. Then generate feature objects as a class-level list as before, but without decoding the region name - just store the region specification in the feature object (typically a string)
• add a _decoded_regions dictionary to the RegionFeature class, which is indexed by the parcellation used. Add the region object obtained by decoding wiwth the given parcellation there.
• Whenever a feature extractor is constructed, go through the already loaded features, check if the regionname had already been decoded by the current parcellation (by looking at the keys of _decoded_regions), and decode only if necessary.

[dickscheid]

This is solved since GH-82